1. Field of the Invention
The present invention relates to an optical head including a plurality of light sources having different wavelengths and adapted to optically record or reproduce information on or from information recording media such as a plurality of types of optical discs, and an optical disc device including such an optical disc.
2. Description of the Background Art
In recent years, Blu-ray discs (hereinafter, BDs) which are high-density and large capacity optical information recording media (hereinafter, also referred to as “optical discs”) having the same size as CDs (Compact Discs) and DVDs (Digital Versatile Discs) have been put to practical use as blue-violet semiconductor lasers have been put to practical use. This BD is an optical disc whose protective substrate is about 0.1 mm in thickness and on or from which information is recorded or reproduced using a blue-violet semiconductor laser beam source having a wavelength of about 400 nm and an objective lens whose numerical aperture (NA) is increased to 0.85.
HD DVDs whose protective substrate is about 0.6 mm in thickness and on or from which information is recorded or reproduced similarly using a blue-violet semiconductor laser beam source having a wavelength of about 400 nm and an objective lens whose numerical aperture is 0.65 have been also put to practical use. These optical discs on or from which information is recorded or reproduced using blue-violet light sources are collectively called high-density optical discs.
There has been proposed an optical head compatible with optical discs whose protective substrates differ in thickness and capable of recording or reproducing information on or from information recording surfaces of these optical discs by focusing laser beams having different wavelengths using one objective lens.
A construction example of such an optical head is shown in FIG. 23. In FIG. 23, an optical head 130 includes a light source 101 for emitting a blue-violet laser beam, a beam splitter 102, a relay lens 103, a hologram element 104, a dichroic prism 105, a collimator lens 106, an objective lens 107, a diffraction grating 108, a two-wavelength light source 111 for emitting a red laser beam and an infrared laser beam, and a light receiving element 112. Further, a BD 60 is an optical disc whose protective substrate is 0.075 to 0.1 mm in thickness.
Here, an operation of the conventional optical head 130 to record or reproduce information on or from the BD 60 is described. A blue-violet laser beam emitted from the light source 101 is reflected by the beam splitter 102 and passes through the relay lens 103 to be converted into a divergent beam having a larger divergence angle. The blue-violet laser beam having passed through the relay lens 103 is converted into a substantially parallel beam by the collimator lens 106 after passing through the hologram element 104 and being reflected by the dichroic prism 105. The blue-violet laser beam converted into the substantially parallel beam is focused as a light spot on an information recording surface of the BD 60 through the protective substrate by the objective lens 107. The laser beam reflected by the information recording surface of the BD 60 passes through the objective lens 107 and the collimator lens 106 again to be reflected by the dichroic prism 105, and passes through the beam splitter 102 to be introduced to the light receiving element 112 after passing through the hologram element 104 and the relay lens 103.
Here, when the laser beam passes through the hologram element 104, a 0th-order diffracted light and ±1st-order diffracted lights are generated. The ±1st-order diffracted lights of the laser beam on the outward path emitted from the light source 101 are blocked by an aperture (not shown) arranged immediately before the objective lens 107 and, therefore, do not reach the BD 60. On the other hand, the ±1st-order diffracted lights of the laser beam on the return path reflected by the BD 60 are detected by specified light receiving areas of the light receiving element 112 and are mainly used for the generation of a servo signal.
Here, a focus error signal can be detected using a so-called astigmatism method of obtaining the focus error signal by a four-divided light receiving pattern after astigmatism is given to a laser beam on the return path, for example, by means of a detection lens (not shown) arranged between the beam splitter 102 and the light receiving element 112. It should be noted that a tracking error signal is described later.
Next, an operation of the optical head 130 in the case of recording or reproduction on or from a DVD 70 which is an optical disc whose protective substrate is 0.6 mm in thickness or a CD 80 which is an optical disc whose protective substrate is 1.2 mm in thickness is described with reference to FIG. 24. A red laser beam having a wavelength of 655 nm or an infrared laser beam having a wavelength of 785 nm emitted from the two-wavelength light source 111 is split into a main beam, which is a 0th-order diffracted light, and sub-beams, which are ±1st-order diffracted lights, by the diffraction grating 108. The main beam and the sub-beams pass through the dichroic prism 105, are converted into substantially parallel beams by the collimator lens 106, and are focused as light spots on an information recording surface of the DVD 70 or CD 80 through the protective substrate by the objective lens 107. The main beam and the sub-beams reflected by the information recording surface of the DVD 70 or CD 80 pass through the objective lens 107 and the collimator lens 106 again, are reflected by the dichroic prism 105, and pass through the hologram element 104, the relay lens 103 and the beam splitter 102 to be introduced to the light receiving element 112 for the generation of an information signal and a servo signal.
Here, focus error signals used for recording or reproduction on or from the DVD 70 and the CD 80 can be detected using an astigmatism method or the like similar to the focus error signal used for recording or reproduction on or from the aforementioned BD 60. Further, a tracking error signal is detected by a so-called three beam method or a differential push-pull method (DPP method) using the main beam and the sub-beams generated by the diffraction grating 108.
It should be noted that the objective lens 107 has a diffraction structure for focusing a blue-violet laser beam for recording or reproduction on or from the BD 60, a red laser beam for recording or reproduction on or from the DVD 70 and an infrared laser beam for recording or reproduction on or from the CD 80 as minute light spots utilizing wavelength differences.
Accordingly, by using such an optical head 130, information can be recorded or reproduced by focusing laser beams having different wavelengths on different types of optical discs by means of one objective lens 107.
Next, the tracking error signal used for recording or reproduction on or from the BD 60 is described in detail.
Japanese Unexamined Patent Publication No. 2004-281026 (prior art 1) discloses an optical disc device for recording or reproduction on or from a high-density optical disc having a plurality of information recording surfaces like the BD 60. FIG. 25 is a schematic construction diagram of another conventional optical head in the case of recording or reproduction on or from a BD.
In an optical head 230 of FIG. 25, a blue-violet laser beam emitted from a light source 201 and reflected by an information recording surface of a BD 60 passes through a beam splitter 202 to be incident on a hologram element 204 after passing through an objective lens 207 and a collimator lens 206, whereby a plurality of beams are generated. The plurality of beams generated by the hologram element 204 are received by a light receiving element 212 after astigmatism is given thereto while passing through a detection lens 203.
The hologram element 204 has seven kinds of areas 204a to 204g to split the incident laser beam into a 0th-order diffracted light and ±1st-order diffracted lights. FIG. 26 is a diagram showing a beam splitting pattern of the hologram element 204 of the conventional optical head 230. The 0th-order diffracted light x0 is generated by the areas 204a to 204g; the +1st-order diffracted light xa by the area 204a; the +1st-order diffracted light xb by the area 204b; the +1st-order diffracted light xc by the area 204c; the +1st-order diffracted light xd by the area 204d; the +1st-order diffracted light xe by the area 204e; the +1st-order diffracted light xf by the area 204f; and the +1st-order diffracted light xg by the area 204g. 
FIG. 27 is a diagram showing a state of a laser beam reflected by the BD 60 and reaching the light receiving element 212 of the conventional optical head 230 with respect to the pattern of the light receiving areas. The light receiving element 212 includes a total of ten light receiving sections 212a to 212j. 
The light receiving sections 212a to 212d are used to detect a focus error signal and a signal used to reproduce information recorded on an optical disc (BD 60).
On the other hand, the light receiving sections 212e to 212j are used to detect a tracking error signal. The optical head can be miniaturized and the number of operation steps upon assembling the optical head can be reduced by forming the light receiving sections 212a to 212d for detecting the focus error signal and the light receiving sections 212e to 212j for detecting the tracking error signal on the same semiconductor substrate.
The light receiving sections 212a to 212j output current signals I212a to I212j corresponding to amounts of received lights. A focus error signal FE is obtained by a calculation: FE=(I212a+I212c)−(I212b+I212d). Further, a tracking error signal TE is obtained by a calculation: TE=(I212e−I212f)−k(I212g+I212h−I212i−I212j).
Here, a 0th-order diffracted light x0 is received by the light receiving sections 212a to 212d; a +1st-order diffracted light xa by the light receiving section 212e; a +1st-order diffracted light xb by the light receiving section 212f; a +1st-order diffracted light xc by the light receiving section 212g; a +1st-order diffracted light xd by the light receiving section 212h; a +1st-order diffracted light xe by the light receiving section 212i; and a +1st-order diffracted light xf by the light receiving section 212j. It should be noted that a +1st-order diffracted light xg is received by none of the light receiving sections. By adopting such a construction, the variation of the tracking error signal occurring when the position, width and depth of a groove formed in the optical disc (BD 60) vary or when information is recorded on a track can be reduced.
This construction also functions to avoid the incidence of unnecessary lights on the light receiving sections used to detect the tracking error signal when the optical disc (BD 60) has a plurality of information recording surface.
The 0th-order diffracted light x0 and +1st-order diffracted lights xa to xg are generated when the laser beam reflected by the information recording surface of the optical disc is incident on the hologram element 204. However, since the optical disc (BD 60) has two information recording surfaces 60a, 60b (not shown), a beam reflected by the information recording surface 60b different from the information recording surface 60a actually used for recording or reproduction is also incident on the hologram element 204 to generate diffracted lights. A 0th-order diffracted light y0 and +1st-order diffracted lights ya to yg are diffracted lights generated when the laser beam reflected by the information recording surface 60b is incident on the hologram element 204. The 0th-order diffracted light y0 is generated by the areas 204a to 204g; the +1st-order diffracted light ya by the area 204a; the +1st-order diffracted light yb by the area 204b; the +1st-order diffracted light yc by the area 204c; the +1st-order diffracted light yd by the area 204d; the +1st-order diffracted light ye by the area 204e; the +1st-order diffracted light yf by the area 204f; and the +1st-order diffracted light yg by the area 204g. 
When the laser beam collected by the objective lens 207 is focused on the information recording surface 60a, there is a large defocus on the information recording surface 60b. Thus, the 0th-order diffracted light y0 and +1st-order diffracted lights ya to yg are also largely defocused on the light receiving element 212. Here, it is designed such that the 0th-order diffracted light y0 and +1st-order diffracted lights ya to yg are incident on none of the light receiving sections 212e to 212j. This is because, if the 0th-order diffracted light y0 and the +1st-order diffracted lights ya to yg are incident on the light receiving sections 212e to 212j, the tracking error signal varies according to the degree of incidence, with the result that a stable tracking control cannot be executed.
The area 204g is defined in a central part of the hologram element 204 shown in FIG. 26, and the +1st-order diffracted light xg generated by the area 204g is not used for the generation of the tracking error signal. Here, the +1st-order diffracted light xg is diffracted in a direction normal to a diffracting direction of the +1st-order diffracted lights xa to xf. Thus, the light receiving sections 212e to 212j can be formed at positions where the +1st-order diffracted light xg will not be incident.
It should be noted that −1st-order diffracted lights formed at positions conjugate with the +1st-order diffracted lights are not incident on the light receiving sections 212e to 212j, either.
International Publication Pamphlet 2004/068480 (prior art 2) discloses an optical head designed to obtain a stable tracking error signal by arranging light receiving sections for 1st-order diffracted lights in such a manner as not to overlap a maximum or minimum range of divergence of a 0th-order diffracted light from a recording layer different from a recording layer, on or from which information is to be recorded or reproduced, for a plurality of types of optical discs each including a multitude of layers.
Japanese Unexamined Patent Publication No. H01-62838 (prior art 3) describes an optical head in which an objective lens and a hologram element are united and this united part of the objective lens and the hologram element is movable independently of a light source. FIG. 28 is a schematic construction diagram showing still another conventional optical head in the case of recording or reproduction on or from a DVD.
In an optical head 330 of FIG. 28, a laser beam emitted from a light source 301 is focused on an information recording surface of an optical disc (e.g. DVD 70) by a hologram element 304 and an objective lens 307. The laser beam reflected by the information recording surface of the DVD 70 passes through the objective lens 307 again and is diffracted by the hologram element 304 while passing the respective areas of the hologram element 304 and is introduced to a light receiving element 312.
An actuator 370 executes a tracking control to drive the objective lens 307 so that a light spot focused by the objective lens 307 follows an information track formed in the information recording surface of the DVD 70 in accordance with a tracking error signal obtained from the light receiving element 312. Here, the hologram element 304 is united with the objective lens 307 and driven together therewith by the actuator 370.
Since the hologram element 304 and the objective lens 307 are united, the laser beam reflected by the DVD 70 hardly moves on the hologram element 304 even if the objective lens 307 moves at the time of the tracking control. Accordingly, even if the optical axis of the objective lens 307 is deviated from that of a laser beam emitted from the light source 301 upon detecting a tracking error signal by a one-beam push-pull method, no offset is generated in the tracking error signal.
Such a construction is useful in the case where the one-beam push-pull method is applied to an optical system, whose information track pitches are relatively larger than light spots and a reflected light from which has a small diffraction angle, such as a DVD-RAM.
Prior art 1 discloses the construction of the optical head capable of detecting tracking error signals suitable for high-density optical discs such as BDs. Prior art 3 discloses the construction of the optical head capable of detecting tracking error signals suitable for optical discs whose information track pitches are relatively larger than light spots such as DVD-RAMs. Further, prior art 2 discloses the construction of the optical head capable of detecting a stable tracking error signal for a plurality of types of optical discs each including a multitude of layers.
However, these prior arts mention neither the beam splitting pattern of the hologram element nor the light receiving pattern of the light receiving element, which enables the detection of tracking error signals suitable for all the types of optical discs in the case of an optical head compatible with the recording or production of both high-density optical discs such as BDs and DVDs and CDs.